1. Field of the Invention
The present invention relates to a semi-crystalline film comprising a melt-stable, biodegradable, lactide polymer composition and a process for manufacturing the film from a melt-stable, biodegradable polymer.
2. Description of the Prior Art
The need for polymeric biodegradable films is well established. Films manufactured from blown or cast processes are well known. Typically in a blown film process, a plastic melt passes through a die which extrudes the molten plastic into an annular shape. Typically, the extruded film is extruded in an upward fashion. As the film moves upward, air is blown into the film which expands the film into a tubular shape. The tube is generally closed at some distance above the die, with a pair of nip rolls.
In a cast film process, a sheet is typically extruded from a slit die. The sheet is thereafter pulled through a series of rollers which cool the extruded sheet and may also elongate the length and width of the sheet to a desired dimension and thickness.
The use of films is widespread and well known in the art. The heaviest use of films occurs in the packaging and disposable article industries. Films employed in the packaging industry include films used in food and non-food packaging, merchandise bags and trash bags. In the disposable article industry, the general uses of films occur in the construction of diapers and personal hygiene articles, including tapes.
In light of depleting landfill space and adequate disposal sites, there is a need for biodegradable films. Currently, films comprising polymers such as polyethylene, polypropylene, polyethylene terephthlate, nylon, polystyrene, polyvinyl chloride and polyvinylidene chloride are popular for their superior extrusion and film-making properties. However, these films are not biodegradable. Furthermore, these films are generally noncompostable, which is undesirable from an environmental point of view.
Films have been developed which are generally considered to be biodegradable. These are films which purportedly have adequate properties to permit them to break down when exposed to conditions which lead to composting. Examples of such arguably biodegradable films include those made from polycaprolactone, starch biopolymers and polyvinyl alcohol.
Although films extruded from these materials have been employed in film containing articles, many problems have been encountered with their use. Often the films are not completely biodegradable or compostable. Furthermore, some biodegradable films may also be unduly sensitive to water, either limiting the use of the film or requiring some type of surface treatment to the film, often rendering the film nonbiodegradable. Others have inadequate heat resistance for wide spread use. Thus, there is a need for a film which is completely biodegradable.
The present invention recognizes the importance of crystallinity and further introduces methods to achieve such crystallinity.
The use of lactic acid and lactide to manufacture a biodegradable polymer is known in the medical industry. As disclosed by Nieuwenhuis et al. (U.S. Pat. No. 5,053,485), such polymers have been used for making biodegradable sutures, clamps, bone plates and biologically active controlled release devices. Processes developed for the manufacture of polymers to be utilized in the medical industry have incorporated techniques which respond to the need for high purity and biocompatability in the final product. These processes were designed to produce small volumes of high dollar-value products, with less emphasis on manufacturing cost and yield.
In order to meet projected needs for biodegradable packaging materials, others have endeavored to optimize lactide polymer processing systems. Gruber et al. (U.S. Pat. No. 5,142,023) disclose a continuous process for the manufacture of lactide polymers with controlled optical purity from lactic acid having physical properties suitable for replacing present petrochemical-based polymers.
Generally, manufacturers of polymers utilizing processes such as those disclosed by Gruber et al. will convert raw material monomers into polymer beads, resins or other pelletized or powdered products. The polymer in this form may then be sold to end users who convert, i.e., extrude, blow-mold, cast films, blow films, thermoform, injection-mold or fiber-spin the polymer at elevated temperatures to form useful articles. The above processes are collectively referred to as melt-processing. Polymers produced by processes such as those disclosed by Gruber et al., which are to be sold commercially as beads, resins, powders or other non-finished solid forms are generally referred to collectively as polymer resins.
Prior to the present invention, it is believed that there has been no disclosure of a combination of composition control and melt stability requirements which will lead to the production of commercially viable, semi- crystalline lactide polymer film.
It is generally known that lactide polymers or poly(lactide) are unstable. The concept of instability has both negative and positive aspects. A positive aspect is the biodegradation or other forms of degradation which occur when lactide polymers or articles manufactured from lactide polymers are discarded or composted after completing their useful life. A negative aspect of such instability is the degradation of lactide polymers during processing at elevated temperatures as, for example, during melt-processing by end-user purchasers of polymer resins. Thus, the same properties that make lactide polymers desirable as replacements for non-degradable petrochemical polymers also create undesirable effects during processing which must be overcome.
Lactide polymer degradation at elevated temperature has been the subject of several studies, including: I. C. McNeill and H. A. Leiper, Polymer Degradation and Stability, vol. 11, pp. 267-285 (1985); I. C. McNeill and H. A. Leiper, Polymer Degradation and Stability, vol. 11, pp. 309-326 (1985); M. C. Gupta and V. G. Deshmukh, Colloid & Polymer Science, vol. 260, pp. 308-311 (1982); M. C. Gupta and V. G. Deshmukh, Colloid & Polymer Science, vol. 260, pp. 514-517 (1982); Ingo Luderwald, Dev. Polymer Degradation, vol. 2, pp. 77-98 (1979); Domenico Garozzo, Mario Giuffrida, and Giorgio Montaudo, Macromolecules, vol. 19, pp. 1643-1649 (1986); and, K. Jamshidi, S. H. Hyon and Y. Ikada, Polymer, vol. 29, pp. 2229-2234 (1988).
It is known that lactide polymers exhibit an equilibrium relationship with lactide as represented by the reaction below: ##STR1##
No consensus has been reached as to what the primary degradation pathways are at elevated processing temperatures. One of the proposed reaction pathways includes the reaction of a hydroxyl end group in a "back-biting" reaction to form lactide. This equilibrium reaction is illustrated above. Other proposed reaction pathways include: reaction of the hydroxyl end group in a "back-biting" reaction to form cyclic oligomers, chain scission through hydrolysis of the ester bonds, an intramolecular beta-elimination reaction producing a new acid end group and an unsaturated carbon-carbon bond, and radical chain decomposition reactions. Regardless of the mechanism or mechanisms involved, the fact that substantial degradation occurs at elevated temperatures, such as those used by melt-processors, creates an obstacle to use of lactide polymers as a replacement for petrochemical-based polymers. It is apparent that degradation of the polymer during melt-processing must be reduced to a commercially acceptable rate while the polymer maintains the qualities of biodegradation or compostability which make it so desirable. It is believed this problem has not been addressed prior to the developments disclosed herein.
As indicated above, poly(lactide)s have been produced in the past, but primarily for use in medical devices. These polymers exhibit biodegradability, but also a more stringent requirement of being bioresorbable or biocompatible. As disclosed by M. Vert, Die Ingwandte Makromolekulare Chemie, vol. 166-167, pp. 155-168 (1989), "The use of additives is precluded because they can leach out easily in body fluids and then be recognized as toxic, or, at least, they can be the source of fast aging with loss of the properties which motivated their use. Therefore, it is much more suitable to achieve property adjustment through chemical or physical structure factors, even if aging is still a problem." Thus, work aimed at the bioresorbable or biocompatible market focused on poly(lactide) and blends which did not include any additives.
Other disclosures in the medical area include Nieuwenhuis (European Patent No. 0 314 245), Nieuwenhuis (U.S. Pat. No. 5,053,485), Eitenmuller (U.S. Pat. No. 5,108,399), Shinoda (U.S. Pat. No. 5,041,529), Fouty (Canadian Patent No. 808,731), Fouty (Canadian Patent No. 923,245), Schneider (Canadian Patent No. 863,673), and Nakamura et al., Bio. Materials and Clinical Applications, Vol. 7, p. 759 (1987). As disclosed in these references, in the high value, low volume medical specialty market, poly(lactide) or lactide polymers and copolymers can be given the required physical properties by generating lactide of very high purity by means of such methods as solvent extraction or recrystallization followed by polymerization. The polymer generated from this high purity lactide is a very high molecular weight product which will retain its physical properties even if substantial degradation occurs and the molecular weight drops significantly during processing. Also, the polymer may be precipitated from a solvent in order to remove residual monomer and catalysts. Each of these treatments add stability to the polymer, but clearly at a high cost which would not be feasible for lactide polymer compositions which are to be used to replace inexpensive petrochemical-based polymers in the manufacture of films.
Furthermore, it is well-known that an increase in molecular weight generally results in an increase in a polymer's viscosity. A viscosity which is too high can prevent melt-processing of the polymer due to physical/mechanical limitations of the melt-processing equipment. Melt-processing of higher molecular weight polymers generally requires the use of increased temperatures to sufficiently reduce viscosity so that processing can proceed. However, there is an upper limit to temperatures used during processing. Increased temperatures increase degradation of the lactide polymer, as the previously-cited studies disclose.
Jamshidi et al., Polymer, Vol. 29, pp. 2229-2234 (1988) disclose that the glass transition temperature of a lactide polymer, T.sub.g, plateaus at about 57.degree. C. for poly(lactide) having a number average molecular weight of greater than 10,000. It is also disclosed that the melting point, T.sub.m, of poly (L-lactide) levels off at about 184.degree. C. for semi-crystalline lactide polymers having a number average molecular weight of about 70,000 or higher. This indicates that at a relatively low molecular weight, at least some physical properties of lactide polymers plateau and remain constant. Sinclair et al. (U.S. Pat. No. 5,180,765) disclose the use of residual monomer, lactic acid or lactic acid oligomers to plasticize poly(lactide) polymers, with plasticizer levels of 2-60 percent. Loomis (U.S. Pat. No. 5,076,983) discloses a process for manufacturing a self-supporting film in which the oligomers of hydroxy acids are used as plasticizing agents. Loomis and Sinclair et al. disclose that the use of a plasticizer such as lactide or oligomers of lactic acid is beneficial to produce more flexible materials which are considered to be preferable. Sinclair et al., however, disclose that residual monomer can deposit out on rollers during processing. Loomis also recognizes that excessive levels of lactide or oligomers of lactic acid can cause unevenness in films and may separate and stick to and foul processing equipment. Thus, plasticizing as recommended, negatively impacts melt-processability in certain applications.
Accordingly, a need exists for a lactide polymer which is melt-stable under the elevated temperatures common to melt-processing resins in the manufacture of film. The needed melt-stable polymer composition must also exhibit sufficient compostability or degradability after its useful life as a film. Further, the melt-stable polymer must be processable in existing melt-processing equipment, by exhibiting sufficiently low viscosities at melt-processing temperatures while polymer degradation and lactide formation remains below a point of substantial degradation and does not cause excessive fouling of processing equipment. Furthermore, the lactide polymer must retain its molecular weight, viscosity and other physical properties within commercially-acceptable levels through the film manufacturing process. The present invention also offers further advantages over the prior art and solves other problems associated therewith.